Anyone who ever has rationalized a big purchase should understand how the Chiefs spent about a half-billion dollars on Patrick Mahomes.

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After leading the Chiefs to their Super Bowl win last season, Mahomes and agency Steinberg Sports went into contract negotiations with the leverage to secure the NFL’s first-ever $200 million contract. It was his turn to stretch the market like successful young quarterbacks before him — and there was nothing the Chiefs could do about it. Until …

Mahomes emerged in July with a 10-year, $450 million extension that, when added to the $25 million in performance incentives and the nearly $28 million remaining over two years on his original contract, could be worth about $503 million through 2031. It includes $141 million fully guaranteed for injury, with $63 million guaranteed at signing.

The extension alone is the biggest contract in the history of North American pro sports — and yet is often interpreted as a bargain for the Chiefs. Former Eagles and Browns CEO Joe Banner sees it as more of a win-win.

“I thought the team did great because they could quantify the cost, and I thought the player did great,” Banner told The Post. “He got a virtually guaranteed 10-year contract in the NFL. That’s unheard of.”

The 25-year-old Mahomes’ contract broke from the established paradigm in terms of both length and bonus structure.

Quarterbacks typically sign extensions under which teams only gain three to four extra years of control. Mahomes only was the sixth NFL player, and the first since 2004, to sign for at least 10 years. There isn’t another active player signed beyond 2026.

The cash flow of Mahomes’ contract is such that he will receive $63.05 million over the first three years — or about $18 million less than on Carson Wentz’s Eagles extension. Mahomes’ three-year snapshot essentially was the equivalent of the maximum value on Teddy Bridgewater’s contract with the Panthers.

“He was trying to find a happy medium between being adequately compensated and trying to leave money for other people to keep the roster around him competitive,” said CBS Sports analyst Joel Corry, a former NFL agent. “As you get late in the contract, it gets hard to get rid of him. That’s what the trade-off is. But there are other ways to do it where he can void out of it and get another bite of the apple.”

Corry said some industry experts believe Mahomes left more than $100 million on the table. But, if Mahomes plays as long as 20-year veteran Tom Brady, he will earn more than $1 billion, Banner estimates.

“People have been looking at basketball for years going, ‘When are the NFL guys going to catch up to basketball?’ ” Banner said. “Then they’re all like, ‘Wait a second, he committed too long.’ He got the right numbers. He got the right structure. When you have a contract that is effectively fully guaranteed, cash flow doesn’t matter anymore.”

Because of the relatively small $10 million prorated signing bonus, Mahomes’ salary cap charges are $5.3 million in 2020, $24.8 million in 2021 and $31.4 million in 2022, allowing the Chiefs to keep intact the majority of a budding dynasty. It’s the “Tom Brady Model” for years with the Patriots.

When the money increases, from 2023-2031, the bulk will be paid as a roster bonus instead of as salary. That anomaly is to Mahomes’ benefit because the specified bonus payment schedule generally requires the Chiefs to commit 18 months in advance to their quarterback. It is more security than if he were reliant on non-guaranteed salary, subject to being cut.

As an example, Mahomes’ $38.9 million roster bonus and $2.5 million salary for 2026 become guaranteed in March 2025. His highest-paying year will be at 32 years old in 2027, when he will be due $59.95 million, including a $49.4 million roster bonus.

Mahomes will receive a $1.25 million bonus every time he wins NFL MVP and an AFC Championship from 2022-31. It is a boatload of money, but the market could pass him by.

Deshaun Watson signed a four-year, $156 million extension after Mahomes and will get another contract in his prime. Dak Prescott, Lamar Jackson or someone else will top Mahomes’ $45 million average annual salary before 2031, especially once the salary cap rebounds from COVID-19 and skyrockets with the injection of new revenue sources.

“I don’t think you are going to get another quarterback to do it,” Corry said. “The Deshaun Watson conventional model, where you give up no more than four new [contract] years, is going to be the trend. He’s not maximizing career earnings by taking on a 10-year extension. It’s never in the player’s best interest to sign extremely long deals.”